The present invention relates to a circuit for processing signals supplied by a zirconium-dioxide oxygen sensor or lambda probe, particularly for use as an interface circuit between such a sensor and an electronic control unit in a system for controlling the emissions of an internal combustion engine for motor vehicles.
More specifically, the invention concerns a circuit including a voltage/current converter with two input terminals for connection to the terminals of the oxygen sensor and an amplifier circuit connected to the output of the converter.
A circuit of this type is constituted by the integrated device LM1964 which is produced and marketed by National Semiconductors. The structure of this known device is described in the article "Instrumentation Amplifier ICs Designed for Oxygen Sensor Interface Requirements" by B. D. Miller and L. R. Sample, IEEE Journal of Solid-state Circuits, Vol. SC-16, No. 6, December 1981.
The known circuit described in this article includes a voltage/current converter in which the input signal from the sensor is processed differentially but the voltage/current conversion is carried out non-differentially with an open loop so that errors which depend on the integration process in the circuit (current and offset bias errors, transistor amplification, Early effect) are introduced. The voltage/current converter is connected to a non-inverting amplifier which further amplifies the errors introduced during the conversion. The amplification characteristics of the integrated circuit LM1964 therefore depend not only on the ratio between the resistances of some resistors, but also on the "matching" of the active components. The device, which has seven terminals or pins for external connections, thus has a good twenty terminals or pins altogether, thirteen of which serve for the calibration of the characteristics of internal generators, the offsets and the gains. The device is expensive and delicate because of the numerous calibration operations needed. Moreover, it requires two different supply voltages (+5 and +12V).